Froth flotation process and apparatus



Aug. 24, 1954 N. A. M LEOD FROTH FLOTATION PROCESS AND APPARATUS 8 Sheets-Sheet 1 Filed Jan. 7, 1952 INVENTOR. a. Whg'ed AW A, m

JTTOR/VEYS Aug. 24, 1954 N. A. MacLEOD FROTH FLOTATION PROCESS AND APPARATUS 8 Sheets-Sheet 2 Filed Jan. 7, 1952 I'LWAr/mv fon bzj-nep JNVENTOR.

mawza gm ATTORNEYS N. A. M OLEOD FROTH FLOTATION PROCESS AND APPARATUS Aug. 24, 1954 8 Sheets-Sheet 3 Filed Jan. 7, 1952 INVENTOR.

1954 N. A. M LEOD 2,687,213

FROTH FLOTATION PROCESS AND APPARATUS Filed Jan. 7, 1952 8 Sheets-Sheet 4 GAS IN V EN TOR.

.954 N. A. M LEOD 2,687,213

FROTH FLOTATION PROCESS AND APPARATUS Filed Jan. 7, 1952 B Sheets-Sheet 5 INVENTOR.

N. A: M LEOD FROTH FLOTATION PROCESS AND APPARATUS Aug. 24, 1954 8 Sheets-Sheet 6 Filed Jan. 7, 1952 s 3X, Illlll'llllw Fllllllll IFIII IIIIIJ JNVENTOR.

flTTOIP/VE'YS Patented Aug. 24, 1954 TENT OFFICE FROTH FLOTATION PROCESS AND APPARATUS Norman A. MacLeod, Altadena, Calif. Application January 7, 1952, Serial No. 265,345

15 Claims. 1

This invention relates to a process and apparatus for concentrating ores, minerals or other materials by froth flotation, in which a body of pulp containing ore or other solids in divided condition, suspended in'water, is subjected to agitation and aerationfor the purpose of forming a multiplicity of small gas bubbles dispersed in the pulp and causing selective attachment of certain solid particles to the gas bubbles. The gas bubbles and attached solid particles rise through the pulp to form a froth containing desired solid constituents, which is removedas a concentrate at the top of the body of pulp.

The term aeration is used herein to mean the distribution of a multiplicity of small bubbles of gas within a body of pulp to permit attachment thereto of certain solid particles whose surfaces may be coated by a suitable agent or com.- bination of agents such as are commonly used in froth flotation. Although air is commonly used for producing these bubbles, it will be understood that any suitable gas may be employed.

This application is a continuation-in-part of my application Serial No. 113,783, filed September 2, 1949.

The principal object of the invention is to provide a froth flotation process and apparatus embodying advantageous steps or means for subjecting the pulp to agitation and aeration and to bubble separation and froth formation, whereby a large quantity of pulp can be treated during continuous flow through a single apparatus, with a highly efiicient formation and recovery of a froth product or concentrate.

A particular object of the invention is to provide a froth flotation process and apparatus in which the agitation and aeration of the pulp and the separation of bubbles to form the froth may be controlled substantialy independently of each other, and in which the pulp is caused to flow alternately through successive zones or regions of agitation and aeration and through successive zones or regions of bubble separation and froth formation above the agitation zones.

A further object is to provide a process and apparatus in which the agitation of the pulp and the formation of the froth may be controlled in such manner as to maintain a different proportion of solids in the pulp during agitation and aeration and during the separation of bubbles to form the froth. A particular object is to provide for maintaining a relatively high percentage of solids in the agitation zone so as to reduce the tendency of solid particles to settle out, and also increase the scrubbing action 'on mineral particles and provide better exposure of the surfaces thereof for contact with and for adhesion to added flotation reagents, and to provide a relatively low concentration of solids in the pulp during the rising of the gas bubbles with attached solid particles and formation of the froth so as to facilitate the rising of the gas bubbles and attached solids and the formation of the froth, and to also promote the gravity settling or dropping out of solid particles that are not attached to gas bubbles. A further advantage that may be gained by maintaining a high proportion of solids in the agitation zone is that desired flotation reagents may be introduced in that zone with resulting improvement in action due to the high concentration of reagent in the liquid phase and its readyavailability to the high proportion of solids present.

A further object is to provide for a general linear flow of pulp through a flotation apparatus from one end to the other, while repeatedly subjecting the pulp during such flow to successive stages of agitation and aeration and to successive stages of bubble separation and froth formation, and to produce active circulation of the pulp in each stage of agitation and aeration, preferably in alocalized path transverse to the direction of the general linear flow of the pulp.

A further object is to provide for agitating the pulp in the presence of air to produce aeration of the pulp and suspension of solid particles therein and to promote contact of the solid particles with the gas bubbles, by withdrawing a portion of the body of pulp at one position by pump action and reintroducing it by the same action into the body of pulp at another position, in such manner and at such velocity as to cause active circulatory motion and agitation in the body of pulp. By changing or properly selecting the relative height of the withdrawal and reintroduction of the pulp, in relation to the overall depth of the pulp, the concentration of solids in the agitation zone may be controlled as desired, which is sometimes of importance for certain purposes.

A further object of the invention is to obtain suspension and agitation of solids in a liquid flotation medium or pulp by continually withdrawing a portion of the pulp, at a position preferably above the bottom of a body of pulp in a tank but below the pulp level, and continually re-introducing the withdrawn pulp forcibly into the pulp body, at or closely adjacent the bottom thereof, preferably substantially parallel to the bottom wall of the tank. In this connection, a particular object is to provide for controlling the concentration of solids in the pulp thus subjected to agitation, by changing or properly selecting the height at which the pulp is withdrawn, with respect to the position at which it is re-introduced and the overall depth of the pulp body.

A further object of the invention is to minimize the consumption of air or other gas by introducing only the amount of gas required for bubble formation and the production of a froth, and providing pump means for effecting active agitation and circulation of the pulp to maintain the solids in suspension and promote contact thereof with the gas bubbles and added flotation reagents.

A further object is to provide for maintaining a high proportion of solids in suspension in an agitation zone in the lower portion of a body of pulp, by the continual withdrawal and forcible reintroduction of pulp, and for maintaining a relatively low proportion of solids in a bubble separation and froth formation zone above the agitation zone, while introducing air into the agitation zone, preferably in the upper part thereof, to cause aeration and rising of gas bubbles with resulting formation of froth in the bubble separation and froth formation zone where the concentration of solid is relatively low.

A further object is to provide an apparatus in which the pulp is cau ed to flow, by gravity, alternately through a plurality of successive agitation zones in the lower part of a horizontally elongated tank and a plurality of successive bubble separation and froth formation zones in the upper part of the tank, and in which the pulp is subjected in each agitation zone to active agitation in the presence of gas to cause aeration of the pulp and contact of solid particles with gas bubbles, and is subjected in each bubble separation and froth formation zone to a minimum amount of agitation other than that caused by the general flow of the pulp longitudinally of the tank, so as to facilitate the rising and s paration of the bubbles and attached solid particles and the settling or dropping out of solid particles that are not attached to gas bubbles.

Another object of the invention is to provide advantageous means for the removal of froth from the froth formation zones, and particularly to provide for the efficient removal of a relatively large quantity of froth per unit length of the apparatus so that. when handling large quantities of pulp in a single apparatus. the froth overflow capacity may be commensurate with the capacity of the apparatus to produce froth by agitation and aeration of the pulp. A. further object in this connection is to provide froth overflow means that have extended overflow lips at the froth overflow level, and that are so arranged as to cause a positive flow of froth laterally toward and over the overflow lips and thus effect complete and positive discharge of a froth of high quality.

Other objects and advantages will be mentioned hereinafter or will be apparent from the following description, reference being made to the accompanying drawings, in which:

Fig. 1 is a partly sectional side elevation of one form of apparatus in accordance with this invention;

Fig. 2 is a partial plan View of the form of apparatus shown in Fig. 1, showing only the portion adjacent the feed end thereof;

Fig. 3 is a transverse vertical section on line 3--3 in Fig. 2;

Fig. 4 is an enlarged fragmentary sectional view showing portions of two adjacent froth discharge troughs, taken on line 4-5 in Fig. 3;

Fig. 5 is a fragmentary horizontal section on line 55 in Fig. 3;

Fig. 6 is a partial longitudinal section on line 6-6 in Fig. 2;

Fig. 7 is an enlarged view on line l--'l in Fig. 3 showing a preferred form of gas delivery pipe in accordance with the invention;

Fig. 8 is a transverse sectional view on line 8-8 in Fig. 7, on a still more enlarged scale;

Fig. 9 is a partial longitudinal vertical section of another form of apparatus in accordance with this invention, taken on line 9-9 in Fig. 11;

Fig. 10 is a partly sectional plan view on line ltlll in Fig. 9;

Fig. 11 is a transverse vertical section on line llH in Fig. 9;

Fig. 12 is a horizontal sectional View on line l2-l2 in Fig. 9;

Fig. 13 is a partly sectional side elevation of another modified form of apparatus according to my invention;

Fig. 14 is a partly sectional plan view of a portion of thi form of apparatus on line l lli in Fig. 13;

Fig. 15 is a transverse sectional view on line l5l5 in Fig. 13, with certain parts shown in end elevation;

Fig. 16 is a schematic longitudinal vertical section of an apparatus of the type shown in Fig. 1, with certain parts omitted, and;

Fig. 1'7 is a schematic view similar to Fig. 16 of an apparatus of the type shown in Fig. 13.

The flotation apparatus shown in Figs. 1 through 8 and illustrated schematically in Fig. 16 comprises a horizontally elongated tank indicated at A, having a bottom wall I, side walls 2 and 3, and end walls 4 and 5 at the feed and discharge ends respectively. One side wall 2 extends vertically from the top of the tank to the bottom wall I. The other side wall 3 extends vertically downward from the top of the tank to a position somewhat above the bottom and is joined to the bottom wall 5 by a wall portion 6 of generally cylindrical curvature which form part of both the side and bottom walls. Above the curved wall portion 6 an auxiliary wall member 5 is provided, extending the length of the tank inside the side wall 3, whose lower portion la is curved or inclined inward and upward as an upward continuation of the curved wall portion 6, and whose upper portion 112 slopes outward and upward to the side wall 3.

The tank A is provided at the feedend with pulp feed means comprising a feed compartment I open at its upper end for introduction of pulp and communicating with the interior of the tank through an inlet opening #2 in the end wall 4, located centrally of the tank and preferably in the lower portion but somewhat above the bottom thereof, as shown in Fig. l. and in dot-dash lines in Fig. 3.

A tailings discharge compartment or launder I3 is provided at the discharge end of the tank, into which the tailings (i. e, the portion of the pulp that is not floated and removed in the froth) pass by overflowing a vertically adjustable gate Ma located adjacent a centrally located tailings discharge opening H1 in the upper portion of end wall 5. Another tailings discharge opening I5 controlled by a vertically adjustable gate lfia may also be provided below the overflow opening I4, for discharging a portion of the tailings from the tank at a lower level into the compartment l3.

The upper edge of the overflow gate i 4a defines a tailings overflow lip located near the top of the tank for maintaining within the tank a pulp level slightly below the froth overflow level L defined by the froth overflow lips described hereinafter. At least part of the tailings are caused to overflow the top of gate I ia, and the desired pulp level may therefore be established and maintained by vertical adjustment of said gate. The term pulp level as used herein means the level at which the unhindered upward movement of individual gas bubbles or aggregates of bubbles due to displacement by the liquid phase substantially ceases, and the bubbles separate from the liquid. Above this level the gas bubbles are collected in the form of a froth, through which liquid drains back into the pulp or liquid phase without producing any appreciable upward displacement of the bubbles.

The construction of my invention is particularly intended and adapted for use in apparatus of relatively large size so as to handle a large quantity of pulp in a single machine, although the invention is not limited to any specific dimensions or proportions. In an apparatus of large capacity, for example, the transverse width of the tank, between the side walls 2 and 3, may be in the neighborhood of ten to twelve feet or more, and the height thereof, from the bottom wall I to the froth overflow level L at the top of the tank, may be in the neighborhood of fifteento eighteen feet or more. Such dimensions make it possible, for example, to treat as much as 30,000 tons of ordinary copper ore per day, resulting in great improvements such as economy of power, increased extraction of valuable minerals, lower labor cost, and lower capital investment. However, machines of smaller size can also be used advantageously for treating smaller tonnages. In order to provide sufficient time for treatment of the pulp and separation of the froth during the generally longitudinal flow of the pulp through the tank from the inlet opening [2 to the tailings discharge openings i4 and [5, the tank may be of any suitable proportionate length, as indicated by the break therein at H5 in Fig. 1. For example, if the transverse dimensions are within the ranges mentioned above for large capacity machines, the length of the tank may be from fifty to two hundred feet or more, depending on the rate of flow of pulp and the treatment time required.

In order to provide a large froth overflow capacity and thus secure a satisfactory quality and rate of separation and removal of froth in a machine of large through-put capacity, the froth discharge means in such large machines preferably comprises a plurality of longitudinally spaced froth-receiving troughs l8 extending transversely of the tank in the upper portion thereof. The troughs l8 are arranged in two sets extending inward fromthe respective sides of the tank and communicate at their outer ends, through openings [9 in the respectiv side walls, With sloping froth discharge launders 21 extending along the sides of the tank. The troughs l8 extend within the pulp in the tank and extend downward to positions below the pulp level. Each froth receiving trough comprises an inner end wall 22, spaced side walls 23 and a bottom wall 24 sloping downward and outward to the bottom of the opening I 9 as shown in Fig. 3.

The side walls of adjacent troughs are spaced from each other to provide passages 25 therebetween for upward movement of gas bubbles through the pulp and formation of froth, as illustrated in Fig. 4 and described more fully hereinafter. Upward movement of gas bubbles and formationof froth is also permitted in the central portion of the tank between the opposing inner ends of the respective sets of troughs, as indicated at 26 in Fig. 3.

The trough side walls 23 terminate at their upper ends in horizontal froth overflow lips 2'! extending transversely of the tank at the froth overflow level L mentioned above, and the removal of froth takes place principally by overflow over these transversely extending lips, although some froth can overflow over the upper edges 22a of the inner end walls 22 and over the upper edges 2a and 3a of the side walls 2 and 3 between adjacent troughs, which are also located at the overflow level L. It will be seen that this arrangement provides a relatively great total length of overflow lips per unit length of the tank, thus providing for the removal of a large quantity of froth at a given desired level.

For most satisfactory separation of gas bubbles and formation and removal of the froth, the lower portions of the side walls 23 of each trough are preferably inclined outward slightly as shown at 23a in Figs. 4 and 6, from the bottom of the trough upward to an intermediate position, and the upper portions thereof are preferably inclined inward slightly as shown at 232), from this intermediate position upward to the overflow lips 21. As a result of this construction it will be seen that the spaced side walls of two adjacent troughs, which define the passages 25 for upward movement of bubbles and formation of froth, converge upwardly from their bottom edges to a somewhat restricted throat 25a intermediate the height of the troughs and below the level of the pulp in the tank, and then diverge upwardly from this restricted throat to the overflow lips at their upper edges.

Within the lower portion of the tank I provide a plurality of longitudinally spaced substantially imperforate transverse lower partitions 3|, each extending across the width of the tank between the side walls and extending upward from the bottom of the tank to an intermediate level spaced a substantial distance below the froth overflow level L. These lower partitions 3| divide the lower portion of tank A into a plurality of successive pulp agitation chambers as indicated at B1, B2, BB-Bx, By in Fig. 1. In Figs. 1, 3 and 6, the upper edges Sla of these lower partitions are shown at an intermediate level below the bottoms of the froth receiving troughs i8 and -a substantial distance below the level of the tailings overflow lip defined by gate Ma. Since the partitions 3| are substantially imperforate below this level, the pulp is caused to pass over the partitions in flowing from each agitation chamber to the next, but the upper edges cm are sufficiently below the pulp level so that this longitudinal movement of the pulp over these partitions occurs principally in the intermediate portion of the tank below the troughs It and hence does not disturb the upward movement of the gas bubbles and the formation and removal of the froth. 7

Within the upper portion of the tank I provide a plurality of complementary substantially imperforate transverse upper partitions 32 that divide'that portion of the tank into a plurality of successive bubble separation and froth formation chambers as indicated at Cr, (Ia-CK, Cy in Fig. 1. These upper partitions are spaced longitudinally of the tank intermediate, and preferably substantially midway between, the lower partitions 3|. Each of the partitions 32 extends across the width of the tank between the side walls and extends from the froth overflow level L downward to an intermediate position, preferably within the agitation chamber between each pair of adjacent imperforate lower partitions. Thus, in Figs. 1, 3 and 5, the lower edges 32a of the-upper partitions are located at a level somewhat below the upper edges 3 Id of the lower partitions. since the partitions 32 are substantially imperforate above this level, the pulp is caused to pass beneath the partitions in flowing from each bubble separation and froth formation chamber to the next, and is thus positively directed into and through a pulp agitation chamber, as described more fully hereinafter. The upper portions 321) of partitions 32 and the upper edge portions of end walls 4 and 5 preferably extend upward a short distance above the froth overflow level L, so as to prevent flow of froth over the tops of these partitions and end walls.

In the particular form of apparatus shown, the upper partitions 32 are spaced at equal intervals between the end walls 4 and 5, except that the interval between the last partition and end wall 5 is only half as great as the other intervals, so that the successive bubble separation and. froth formation chambers C1, C2-C5; are of equal length in the direction of general longitudinal fiow of the pulp, but the last chamber Cy is half the length of the others. The first of the series of lower partitions 3| is disposed respectively midway between the first upper partition and end wall 4, and the remaining lower partitions are spaced equally between this first partition and end wall 5, midway between the successive upper partitions, so that the first pulp agitating chamber B1 is half the length of each of the remaining chambers BzB If the transverse dimensions of the tank are within the ranges given above, for example, the length of each of the half-length chambers B1 and Cy may be about five to six feet or more, and the length of each of the other pulp agitation and froth separation chambers may be about ten to twelve feet or more.

The apparatus may be considered as comprising a plurality of successive sections or cells whose upper portions are separated by the partitions 32. Thus, the upper portion of each section forms a corresponding bubble separation and froth formation chamber such as C1, while the lower portion of each section except the last is divided by the corresponding lower partition 3| into two agitating zones each half the length of the section, the adjoining agitating zones of each two adjacent sections being in open communication with each other below one of the partitions 32, to form a single agitation chamber such as B2.

The apparatus further comprises means for agitating the pulp in each agitation chamber in the presence of air or other gas, to thoroughly aerate the pulp. The pulp agitating means preferably comprises suitable pumps including at least one pump for each agitation chamber, connected thereto in such manner as to withdraw pulp therefrom and to return it thereto at such position and velocity and in such direction as to maintain active circulation of the pulp in the chamber and suspension of the solid particles in the liquid.

As shown particularly in Figs. 2 and 3, this particular form of apparatus is provided with a series of pumps arrangedv along one side of the tank, outside the side wall 2, the first two such pumps being indicatedat 35 and 36 in Fig. 2. The first pump 35 is connected to the first agitation chamber B1. The second pump 36 is connected to the first half of the second agitation chamber B2, between the first pair of partitions 3| and 32. It will be understood that a similar pump is connected to the second half of the second chamber B2, between the first partition 32' and the next partition 3|, and that two such pumps are similarly provided for each succeeding agitation chamber Bil-B All the pumps are connected in the same manner to the corresponding agitation chambers, and the following description of pump 35 and its associated piping and connections will therefore apply also to each of the other pumps.

The pump 35 is connected to agitating chamber B1 by a pipe system through which it continually withdraws pulp therefrom at one position and returns it thereto at another position. This pipe system comprises a pulp withdrawal line 38 connected to the pump inlet and connected to the tank A by two flared branch connections 39 opening into the chamber B1 through pulp withdrawal openings 38 in the side wall 2 located at a substantial distance above thebottom of the chamber and preferably in the upper portion thereof but a substantial distance below the level of the tailings overflow lips defined by gate Ma. As shown in Fig. 1, the openings 43 are elongated horizontally and longitudinally of the tank so as to with draw pulp from substantially the entire length of the chamber B1. In the particular construction shown, the openings 43 are located at approximately the same level as the lower edges of the upper partitions 32. The pulp withdrawal line 38 is provided with a valve M.

The pipe system of pump 35 further comprises a pulp return line 42, also provided with a valve 33, connected to the pump outlet and connected to the tank A by two fiared branch connections 44 opening into the lower portion of chamber B1 through pulp return openings 35 in the side wall 2, preferably in a substantially horizontal direction transversely of the tank and closely adjacent the bottom wall so as to direct the returned pulp across the bottom of the chamber and toward the curved wall portion 65 at the opposite side of the chamber. The openings 45 are also elongated horizontally and longitudinally of the tank so as to introduce pulp into substantially the entire length of chamber B1.

Gas supply pipes 43 and 4?, controlled by valves Mia, and Ma and connected to a source of compressed air or other gas at suitable pressure, are shown connected to the pulp withdrawal and return lines 38 and 42 respectively, for introducing. gas into the pulp either ahead of or beyond the pump or both I also prefer to provide means for introducing any desired flotation agents into the pulp in each agitation chamber, and such means may advantageously comprise a flotation agent supply pipe connected to the piping system of each pump. For example, I have shown in Fig. 3 a pipe 43, controlled by a valuve 38a connected to the pulpreturn line 52 of pump 35. The pipe 43 may be connected to a source of supply of a flotation agent or agents in liquid form or in solution in water, at suitable pressure to overcome the pump outlet pressure and thereby continually introduce a controlled proportion of the flotation agent into the pulp in line 42. If desired, this pipe 48 may also be connected to a source of water'underpressure, whereby a stream of water may be introduced at high velocity into chamber B1 through the pulp return openings 45, for the purpose of dislodging and suspending solids that may settle and accumulate at the bottom of the chamber in the event of a prolonged shutdown while the tank is full of pulp.

Pipes 4! and 5E? having normally closed valves 49a and 56a, may be provided interconnecting the pulp withdrawal lines 33 and the pulp return lines 42, respectively, of each pair of adjacent pumps. If it is desired to disconnect any one of the pumps for repair or other purpose, the valves 4| and 43 in the lines of that pump may be closed, and the valves 49a. and 55341 in the interconnecting pipes to an adjacent pump may be opened, so that such adjacent pump will then recirculate the pulp in the agitation sections normally served by both pumps. These interconnecting pipes also make it possible, if desired, to operate the pumps in such maner as to transfer part of the pulp withdrawn from one section of the agitation chambers to an adjacent section either upstream or downstream therefrom with respect to the general linear flow of pulp longitudinally of the apparatus. For example, if either valve 4| or 43 in the lines of one pump is partially closed and the valve 49a or 50a in the corresponding connecting pipe to an adjacent pump is partially opened, the adjacent pump will either draw part of the pulp from the pulp withdrawal line 33 of the one pump and transfer it to the agitation section corresponding to the adjacent pump, or else deliver to the pulp return line 42 of the one pump part of the pulp withdrawn from the section corresponding to the adjacent pump.

As indicated in Fig. 1, all of the agitation chambers are provided with pulp withdrawal openings 46 and pulp return openings 45, as described above, and it will be understood that these openings are connected to the corresponding pumps in the same manner as described for chamber B1. For example, it will be apparent from Figs. 1, 2 and 5 that the pulp withdrawal and return openings of the first half of the second chamber 132 are similarly connected to the corresponding pump 36.

The continual withdrawal and reintroduction of pulp by the pumps and piping system described above maintains active circulation and agitation of the pulp in each agitation chamber. The circulatory movement of pulp within each chamber is principally in generally circular or annular paths transverse to the length of the tank, as indicated by the curved arrows in the chamber B1 in Fig. 3, a portion of the circulating pulp being continually withdrawn through openings 4|] and returned through openings 45. Thus the pulp circulates about an axis extending longitudinally of the tank and located substantially centrally of the agitation chambers, and the inner surface of wall portion 6 is preferably curved cylindrically about this central longitudinal axis of circulation.

Above the agitation chambers but below the pulp level in the tank and at a substantial distance below the froth overflow level L, I provide a plurality of spaced vertically extending baflies 5|. These bafiies, which are preferably disposed in the lower portion of each bubble separation and froth formation chamber such as 0;, below the froth overflow troughs I8 and above the lower transverse partitions 3|, extend longitudinally of the tank A and perpendicular to the planes of the above described circulatory movement of the pulp in the agitation chambers. Gas bubbles with attached solid particles pass upwardly through the pulp in the spaces 52 between bafiles 5| to form a froth in the upper portions of the bubble separation and froth formation chambers. The baflles intercept the circulatory motion of the circulating pulp in its path transverse to the length of the tank, so as to maintain a relatively quiescent condition in the liquid between and above the baffles and thereby facilitate rising of bubbles and formation and removal of froth. The lower edge portions 5|a of some of the baifies -5| are shown as being curved or inclined in a downward direction toward the side of the tank at which the pulp is withdrawn and returned by the pumps, so as to be substantially perpendicular or normal to the annular paths of circulator motion of the pulp, and thus minimize the tendency for the circulating pulp to be diverted upward by the baffles. Since the baffles extend longitudinally of the tank, they do not interfere with or obstruct the longitudinal flow of pulp through the bubble separation and froth formation chambers and above the partitions 3|, in passing from one agitation chamber to the next.

The apparatus also includes means for introducing air or other gas into the pulp at a plurality of distributed positions in the tank at an intermediate level therein, at a substantial distance below the pulp level and preferably in the upper portion of each agitation chamber B1, B2 etc., under sufficient pressure to cause gas bubbles to enter the pulp. Such gas introducing means is shown as comprising a plurality of transversely spaced gas delivery pipes 54 extending longitudinally across substantially the entire length of the agitation chambers. In the particular construction shown, a set of gas delivery pipes 54 is provided beneath each bubble separation and froth formation chamber, below the longitudinal baffies 5| and adjacent but slightly above the level of the pulp withdrawal openings 40 and the lower edges of the upper partitions 32. Each set of pipes 54 extends from one of the upper partitions 32 to a position adjacent the next upper partition or one end wall of the tank, passing through openings in the intervening lower partition 3|. The openings in the lower partition may if desired be sealed in any suitable manner to prevent undesired passages of pulp therethrough. The pipes 54 of each set are distributed across substantially the entire width of the agitation chambers served thereby, as shown in Fig. 3.

Each of the gas delivery pipes 54 is provided with a plurality of small gas outlet openings 55 spaced at suitable intervals along the length thereof, and is connected at one end by a vertical pipe 56 extending upward to a position above the top of the tank, where it is connected through a control valve 56ato a gas supply pipe 5| extending across the top of the tank. The several gas supply pipes 51 are connected by a pipe 58 to a main gas supply line 59 which is preferably provided with a shut-01f valve 60 and is connected to a source of air or other gas at suitable pressure, such as a compressor or receiver, not shown.

A preferred construction of the gas delivery pipes in accordance with my invention is illustrated particularly in Figs. 7 and 8, in which the pipe 54 is formed of a section or length of cylindrical tubing of rubber or other resilient and flexible material which is distensible or deformable outwardly by internal gas pressure. The length of tubing has an internal cylindrical bore 52 and is secured tightly at one end, as by clamp 63, to a nipple or other fitting at the lower end of the vertical pipe 56, the other end of the tubing being closed by a plug or cap 64 (Fig. 1) clamped or otherwise secured thereto. The resilient pipes may be supported by any suitable means such as U-shaped straps 65 supporting each pipe at suitable intervals and secured to supporting bars 66 which are supported at their ends on the side walls of the tank.

The gas outlet openings 55 are provided at suitable intervals along the length of the pipe, and are preferably arranged in two rows at the opposite sides of the cylindrical wall Bl. The openings 55 are quite small, preferably so small as to be substantially closed in the absence of internal pressure, and are adapted to be enlarged significantly upon application of gas under pressure to the internal bore 62, due to outward deformation or distension of the resilient material of which the wall 6? is formed and also so small that the formation of small gas bubbles is promoted. Such openings may be formed, for example, by piercing the wall of the tube with a sharp pointed instrument, and are preferably small enough to result in formation of small gas bubbles even when enlarged significantly by the internal pressure. In order to cause increased outward deformation, under internal pressure, of the portions of the tubing wall immediately adjacent the openings 55, I prefer to form the wall 6'! with an internal cupshaped recess 53 surrounding each opening, so as to provide a marginal wall portion 89 immediately adjacent and around the opening, of materially less thickness than the remaining portions of the wall between these marginal portions. The internal gas pressure causes the marginal wall portion 69 to bulge outward and form a protuberance of generally spheroidal shape, as indicated in dot-dash lines at 69' in Fig. 8, having a radius of curvature considerably less than the radius of the tube, thus causing a substantial further decrease in thickness of this part of the wall and a substantially increased enlargement of the opening 55, as indicated at The perforated gas delivery pipes 54 introduce gas into the pulp in the upper portion of each pulp agitating chamber, forming a multiplicity of small gas bubbles, and the agitation of the pulp causes these bubbles to be thoroughly distributed throughout the circulating pulp and brought into intimate contact with solid particles, with resulting attachment of those particles that tend to attach themselves thereto. The gas bubbles with the attached solid particles eventually rise to the top of the agitation zone and pass upwardly between the baiiles i and through the overlying bubble separation and froth formation zone, where they accumulate to form a froth at the top of the pulp, and overflow the lips 21 into the troughs it. The operation of this and other forms of apparatus shown will be described more fully hereinafter.

The modified form of apparatus shown in Figs. 9 through 12 comprises a tank A generally similar to the tank A described above except for the cross-sectional shape of the lower portion. The

tank A has an upwardly and inwardly concavecurved lower wall portion H forming the bottom wall and the lower portions of the side walls. The wall portion l! is generally semi-cylindrical in shape, and is joined at its upper edges to vertioal side walls 2 and 3 which extend up to the froth overflow level L. Auxiliary wall members 'i extend the length of the tank within the side walls above the curved wall portion H, with their lower portions la inclined or curved inward and upward as upward continuations of the curved wall portion and their upper portions 11) sloped outward and upward to meet the side walls 2' and 3', as shown in Fig. 11. Tank A further comprises an end wall A at the feed end, and is provided with a pulp feed compartment ll communicating with the lower portion of the tank through an opening l2 in said end wall.

Only a portion of the apparatus adjacent the feed end is shown in the drawings, but it will be understood that the tank A may be of any desired length and is provided at the other end with another end wall and with tailings discharge means including means defining a tailings overflow 'lip, all as described above except that the other end wall is also modified in shape to conform to the cross-sectional shape of the tank.

The above comments regarding the number of successive chambers, the proportions of these chambers, the ability to handle a large quantity of material in a single machine, and the examples of dimensions that may be used in a large capacity machine, are also applicable to this form of apparatus.

The tank A is shown as provided with froth overflow means. similar to those described above, including transverse troughs 48 in the upper portion of the tank and side launders 2!, and is also similarly provided with a plurality of longitudinally spaced lower partitions 3i dividing the lower portion of the tank into a plurality of successive pulp agitation chambers such as B1 and B2, and with a plurality of longitudinally spaced upper partitions 32' dividing the upper portion of the tank into a plurality of successive bubble separation and froth formation chambers such as 01' and C2. I'he lower edges 32a of the upper partitions extend somewhat below the upper edges 31a of the lower partitions, and the arrangement of these partitions is the same as described above except that the shape thereof is modified to conform to the shape of the tank.

In this form of apparatus the pulp circulating means comprises a series of pumps, preferably located above the tank A as shown, with piping systems connected to the agitation chambers in such manner as to continually withdraw pulp therefrom and return it thereto, to maintain active circulation and agitation of pulp therein. At least one pump is provided for each agitation chamber. In this case also, the arrangementof the partitions 3! is assumed to be such that the first agitation chamber B1 is half the length of the remaining chambers, as described above. I have shown two pumps 15a and 7512 connected to the first chamber B1. Four pumps may be provided for each remaining agitation chamber, the first two of these pumps for the chamber B2 being shown at 56c and 16b in Fig. 9. All of these pumps are connected to the corresponding agitation chambers in the same manner as described below for the pumps 15a and 15b.-

The pipe system for each pump 15a and 157 comprises a pulp withdrawal line 18 connected to the pump inlet and extending downwardly into the tank and opening into the pulp agitation chamber B1 at a substantial distance above the bottom thereof, through pulp withdrawal openings 19a at the lower ends of two downwardly and outwardly inclined branch pipes 19 connected to the lower end of each line 18. The pulp withdrawal openings 19a preferably open into the upper portion of the agitation chamber, below the overlying bubble separation and froth. separation chamber, and I have shown means for adjusting the level thereof, comprising an outer pipe section or sleeve 18a slidably connected. to an inner pipe section 78b in each line 18, the branch pipes 19 being connected to sleeve 18a by pipe 180. The section "i817 extends downwardly within the sleeve 18a and suitable means such as a packing gland is provided for maintaining a liquid tight seal between section 18b and the upper end of sleeve 18a in all positions of vertical adjustment thereof. Sleeve 18a may be held in any desired position by means of a set screw 80. In the position shown, the pulp withdrawal openings 19a are located approximately at the level of the lower edges 32a of the upper partitions 32, but they may be adjusted to higher or lower positions by movement of sleeve 18a, so as to vary the level at which pulp is withdrawn from chamber B1. However, the level of pulp withdrawal is in all cases below the bubble separation and froth formation chambers, and a substantial distance below the level of the tailings overflow lip.

The pipe system for each pump 15a and 15b further comprises a pulp return line 8| connected to the outlet of the pump and extending downwardly within the tank and provided at its lower end with two flared oppositely and outwardly directed delivery members 82 having pulp return openings 82a at their outer ends, opening into the lower portion of chamber B1, preferably closely adjacent the curved bottom Wall H and in directions substantially paralle1 thereto. The pulp return openings 82a are elongated horizontally and longitudinally of the tank so as to introduce pulp into substantially the entire length of chamber B1, in outward directions at opposite sides of the center thereof and toward the respective side portions of curved wall I l.

The lines 13 and 8| are provided-With valves 84 and 85 respectively. The pulp withdrawal lines 18 and the pulp return lines 8! of each pair of adjacent pumps are interconnected by pipes 36 and 87, respectively, provided with normally closed valves 86a and 81a, in the same manner and for the same purpose as described above in connection with Figs. 1 to 8. It will be understood that these lines may also be provided with valved connections for introducing air or other as, flotation agents and/or water thereto, in the same manner as described in connection with Figs. 1 to 8.

The arrangement of the pulp circulating means in this form of apparatus is such as to maintain active circulation of the pulp throughout each agitation chamber, due to the continual withdrawal of pulp by each pump through the corresponding pair of openings 19a in the upper portion thereof and the continual reintroduction of pulp, through the corresponding pair of pulp return openings 82a, outwardly and upwardly at the respective sides of the tank. The circulatory movement of pulp thus maintained is principally in two generally circular or annular paths transverse to the length of the tank and within the respective side portions thereof and about two axes 14 extending longitudinally of the tank, as indicated by the curved arrows in the chamber B1 in Fig. 11.

This apparatus is also shown as provided with a set of vertical baffles 9| extending longitudinally of the tank and transverse to the circulatory motion of the pulp, above the agitation chambers but below the pulp level and at a substantial distance below the froth overflow level L. These baffles are spaced from each other and from the side walls of the tank to provide passages 92 therebetween permitting upward movement of gas bubbles with attached solid particles and also permitting movement of pulp longitudinally of the tank from one agitation chamber to the next. The general arrangement and purpose of these bailles is comparable to the baffles 5| in the first described form of apparatus, and some of them are shown as similarly provided with curved lower edge portions Sta directed substantially perpendicular to the paths of circulatory movement of the pulp, for minimizing the tendency to divert the circulating pulp upwardly therebetween.

The apparatus is also provided with means for introducing gas into the pulp in the upper portion of each pulp agitation chamber, including perforated gas delivery pipes 54 constructed and arranged in substantially the same manner as previously described, and identified by the same reference characters.

Figs. 13 to 15 illustrate another modified form of apparatus, in which the pulp in each agitation chamber is caused to circulate about a vertical axis instead of a horizontal axis, the circulation being in this case also produced by continually withdrawing and returning portions of the pulp by the action of pumps, preferably located outside the tank as shown.

Referring to these figures, the upper portion of the horizontally elongated tank D is of rectangular shape, having vertical side walls 94 and 95 and end walls 96 and 91, and this upper portion of the tank is divided by longitudinally spaced transverse vertical upper partitions 98 into a plurality of bubble separation and froth formation chambers or zones such as indicated at F1, F2, F3Fx. The lower portion of the tank is formed by a plurality of contiguously arranged agitation sections having vertically extending lower wall portions 99 of cylindrical shape whose diameter is approximately equal to the width of the upper portion of the tank and also equal to the spacing between adjacent upper partitions 98, and suitably formed transformation portions Iii! joinin the cylindrical portions 99 to the rectangular upper portion of the tank. The bottom walls I02 of these agitation sections combine to form the bottom Wall of the tank and are preferably provided with curved inwardly concave peripheral portions 182a joining the cylindrical Wall portions 99.

These generally cylindrical agitation sections are arranged with the upper edges of the transformation portions IGI tightly joined to the side walls 94 and 95 and, in the case of the two end sections to the end walls 96 and 91 respectively, and thus form part of the side and end walls of the tank. The tank is provided at one end with a pulp feed compartment I04 opening into the upper portion of the first agitation section through an opening I05 in one end wall of the tank, and is provided at the other end with openings I06 and Isl, having adjustable gates Iota and l 01a, for discharging tailings into a tailings discharge compartment or launder I98. As before, the upper edge of gate 106a defines a tailings overflow lip near the top of the tank for maintaining the desired pulp level in the upper portion .of the tank, adjacent but slightly below the froth overflow level L".

The walls .01 adjacent agitation sections are tightly joined together at their upper edges as indicated at I09, and form lower transverse partitions as indicated at III, dividing the lower portion of the tank into a plurality of successive pulp agitation chambers E1, E2, E3-Ex. These lower transverse partitions are preferably extended upward above the lower edges 98a of the upper partitions 98, by means of auxiliary partition members or portions I I lb extending transversely between the side walls 94 and 95, and joined to the upper edges of the adjoining agitation sections and extending upwardly therefrom. The upper edges IIIa of the lower partitions I II are thus somewhat above the lower edges of the upper partitions, but a substantial distance below the froth overflow level indicated at L", and also a substantial distance below the pulp level in the tank so as to permit longitudinal movement of pulp over the top of the lower partitions, through the bubble separation and froth formation chambers, from one agitation chamber to the next.

The upper partitions 98 are located above and at approximately the mid-length of the respective pulp agitation chambers and extend diametrically with respect thereto, the lower edges 98a. of said partitions being spaced a substantial distance above the bottom walls I02 of the agitation chambers, so as to permit a general longitudinal movement of pulp beneath the upper partitions, through the agitation chambers, from one bubble separation and froth formation chamber to the next, and to also permit active circulating movement of pulp in each agitation chamher. The upper edges of partitions 98 and end walls 96 and 91 preferably extend somewhat above the froth overflow level L".

In this form of apparatus, the pulp agitation chambers are all of equal length, as determined by the diameter of the cylindrical lower wall portions 99, while the first and last froth separation chambers F1 and E; are each half the length of the intervening chambers such as F2 and F3. It will be understood that the tank may be of desired length and transverse dimensions, as indicated by the break therein at H5, and may be provided with any desired number of successive agitating chambers and bubble separating and froth formation chambers. This apparatus is also particularly advantageous tor the treatment of a large amount of material in a single machine, and the above comments as to examples of dimensions that may be employed in a large capacity machine are also applicable to this form of apparatus.

This apparatus is also shown as provided with froth overflow means like those above described, including spaced transverse troughs it in the upper portion of the bubble separation and froth formation chambers, and side launders 2!, the overflow lips 2'! of said troughs being at the froth overflow level L."

The means for circulating and agitating the pulp in each agitation chamber comprises a series of pumps, which are shown as located outside the tank along one side thereof. At least one such pump is provided for each agitation chamber, and I have showntwo pumps III and HS for each chamber, each pump being connected to the agitation chamber by a piping system for continually withdrawing and returning portions of the pulp. The following description refers particularly to the pumps and piping systems for the first agitation chamber E1 and it will be understood that the pumps and piping systems for the other agitation chambers are similar and are connected in the same manner to the corresponding chambers.

The piping system for pump II'I comprises a pulp withdrawal line H9 connected to the pump inlet and extending into the tank and opening into the agitation chamber E1 at a substantial distance above the bottom thereof and preferably in the upper portion of the agitation chamber, through a pulp withdrawal opening I2I. The pulp withdrawal opening I25 is located adjacent the periphery of the upper portion of chamber E1, in a direction generally tangential to the periphery of the cylindrical wall portion Q9. The outlet of pump II! is connected to a pulp return line I22 which also extends through a wall tank and opens into the chamber E1 through a pulp withdrawal opening I23 closely adjacent the bottom wall H12, in a direction substantially tangent to a circle about the vertical axis of a cylindrical wall portion 99 at the position of said opening. The pulp return opening I23 is preferably located in the outer portion of the chamber E1 at a substantial distance from the axis thereof, and is shown as located just inwardly of the curved bottom wall portion "320.. The piping system for the other pump H8 similarly comprises pulp withdrawal and pulp return lines I24 and I25 connected to the pump inlet and outlet respectively and extending within the tank and opening into chamber E1 through pulp withdrawal and pulp return openings I26 and I2? respectively. The location and direction of the openings I25 and I2? are similar to those of openings I2I and I23, except that they are located somewhat inwardly from the respective openings I2I and I23 so as to withdraw and return portions of pulp at positions nearer the vertical axis of the generally cylindrical agitation chamber. The pulp withdrawal openings I2i and I28 are located below the bubble separation and froth formation chambers, at a substantial distance below the level of the tailings discharge lip.

The pulp withdrawal and pulp return lines for each pump are provided with valves as indicated at IIta, I22a, IBM, I25a. The two pulp withdrawal lines IE9 and IN are interconnected by a pipe I28 provided with a valve I28a, and the two pulp return lines I22 and I25 are interconnected by a pipe I28 provided with a valve I29a, in the same manner and. for the same general purposes as described above in connection with the interconnections between adjacent pairs of pumps in Figs. 1 to 8.

Gas supply pipes I3I, I32, I33 and I34, provided with valves I3Ia, I32ct, I33a and I3tu. are connected to a source of compressed air or other gas and to the lines IIG, I24, I22 and I25, re-

, spectively, for introducing gas into the pulp in these lines, either ahead or beyond the pumps or both.

Pipes I35 and I38 provided with valves I351; and itlia are also connected to the pulp return lines I22 and I25 respectively, for introducing suitable flotation agents into these lines as described above, or for introducing water under pressure for the purpose of discharging and suspending solids that are accumulated in the bottom of the chamber E1 in the event of a pro-- 17 longed shut down while the tank is full of pulp.

It will be understood that either or both of the pumps II! and H8 may be operated continually to withdraw pulp from the agitation chamber and return it forcibly thereto, and that the recirculation of the pulp in this manner by either or both of the pumps will maintain effective agitation of the pulp in each agitation chamber by active circulatory movement, principally in generally circular or annular paths about the vertical axis of each chamber, as indicated by the curved arrows in the chamber E1 in Fig. 14, a portion of the circulating pulp being continually withdrawn through openings I2I and/or I25 and returned through openings I23 and/or I21. Since the pulp return openings I23 and I2! are located at the bottom of the chamber and are directed substantially. parallel to the bottom wall I02, the entire body of pulp in the agitation chamber is kept in motion at sufficient velocity to prevent settling and accumulation of solids at the bottom of the tank and to maintain the solid particles in suspension in the pulp.

The apparatus is also provided with a set of vertical baffles located below the pulp level in the tank, immediately above each of the pulp agitation chambers and at a substantial distance below the froth overflow level L", which cooperate with the upper transverse partitions 98 to prevent the above described circulatory movement of the pulp from persisting upwardly within the bubble separation and froth formation chambers, while permitting upward movement of gas bubbles with attached solids and also permitting movement of the pulp between these bafiies in passing successively into and through the alternately arranged pulp agitation chambers and bubble separation and froth formation chambers. Each of these sets of bafiles preferably comprises a plurality of longitudinally spaced bafiles I II extending transversely of the tank, and a plurality of transversely spaced bailies I42 extending longitudinally of the tank, in the lower portion of each bubble separation andfroth formation chamber F1, F2, et cetera. The upper edges of these baffles are spaced below, the bottom of the froth overflow troughs I8, preferably at ap proximately the same level as the upper edges I I la of the lower transverse partitions II I.

The apparatus is also provided with means for introducing gas into the pulp in the upper portion of each pulp agitation chamber, including perforated gas delivery pipes I44 extending across the agitation chamber and spaced from each other, and each provided with a plurality of spaced small gas outlet openings I45. The pipes I44 are shown as extending longitudinally of the tank, each pipe being closed at both ends as at I46 and being connected at an intermedi ate point to a vertical pipe I 41 extending upward to a position above the top of the tank, where it is connected through a control valve I 47a to a gas supply pipe I48 extending across the top of the tank. The several gas supply pipes I48 are connected by pipe I49 to a main gas supply line I5I which is preferably provided with a shut off valve I52 and is connected to a source of air or other gas at suitable pressure.

The operation of the above forms of apparatus will be understood from the above description and from the following description of the process according to my invention as carried out 18 ratus and also to the schematic showings in Figs. 16 and 17.

Fig. 16 is a somewhat schematic longitudinal vertical section of an apparatus of the type shown in Figs. 1 to 8, illustrating the general linear flow of pulp therethrough, the circulatory movement of pulp in the agitation chambers and the rising of bubbles to form a froth in the bubble separation and froth formation chambers. For the sake of simplicity of illustration, the froth overflow troughs I8 and the longitudinal bafiles 5| are omitted in Fig. 16, and I have shown only three successive agitation chambers B1, Bzand B3 and three successive bubble separation and froth formation chambers C1, C2 and C3. These chambers are arranged in the same manner as in Fig. 1 except that the last chambers B3 and C3 in Fig. 16 correspond to the chambers By and Cy in Fig. 1.

The pulp, to'be subjected to flotation, and comprising finely divided solids and water and ordinarily containing one or more frothing agents, collecting agents and/or other agents such as are commonly employed in froth flotation, is introduced into one end of the tank through the feed compartment I I and opening I2 into the tank A, and is caused to flow by gravity in a generally linear movement longitudinally of the tank, passing alternately through the successive agitation zones and bubble separation and froth formation zones, in a sinuous path extending above the lower partitions 3| and beneath the upper partitions 32, as indicated by the series of spaced in such apparatus, reference being had to the drawings showing the difieernt forms of appaarrows at P.

The imperforate lower partitions 3I prevent direct flow of pulp from each agitation chambe to the next, but provide an opening extending acres the full width of the tank above each partition, from the top edge 3Ia thereof to the top of the tank, permitting longitudinal movement of pulp within the corresponding bubble separation and froth formation zone. In a similar manner, the imperforate upper partitions 32 prevent direct flow of pulp from each bubble separation and froth formation chamber to the next, but provide an opening extending across the full width of the tank below each partition, from the bottom edge 32a thereof to the bottom of the tank, permitting longitudinal movement of pulp with the corresponding agitation chamber. The pulp ilow openings thus provided above the lower partitions and below the upper partitions may be unobstructed, as shown in Fig. 1 and in chambers C1 and B2 in Fig. 16. However, it is within the scope of my invention to provide additional bafiie means for partially restricting the flow of pulp through these openings so as to modify or control the distribution of the flow of pulp therethrough. For example, baffle means in the form of a perforate partition 3 lb having spaced perforations 3Ic may be provided above any one or more of the lower partitions 3I, as illustrated in the case of the second partition 3| in Fig. 16, to partially restrict longitudinal movement of pulp through the corresponding bubble separation and froth formation chamber, such as C2 in this case, and thus cause the longitudinal flow to be distributed more evenly throughout the height of that chamber above the imperforate partition 3|. Similarly, baffle means in the form of a perforate partition 3212' having spaced perforations 320 may be provided below any one or more of the upper-partitions 32, as illustrated in the case of the second partition 32 in Fig. 16, to partially restrict longitudinal movement of pulp through 19 the corresponing agitation chamber, such as B; in this case.

The pulp entering each agitation chamber is agitated and circulated therein in annular paths generally transverse to the length of the tank as described above and as indicated by the arrows at R in chamber B1 in Fig. 16, and is brought into intimate contact with gas bubbles formed by introduction of gas into the pulp within the agitation chambers as described above, to cause selective attachment of certain solid particles to the gas bubbles. The gas is supplied principally through the gas delivery pipes 1, but gas may also be supplied to the pulp circulating lines of the pumps, as previously described, and additional flotation agents may also be introduced into these circulating lines as also described above. This circulatory movement of pulp in each agitation chamber is superimposed upon the general linear flow of pulp therethrough, so that the actual motion of the pulp is the resultant of the two components indicated by the solid arrows P and the dotted arrows R.

Some of the gas bubbles carrying attached solid particles rise upwardly out of the agitation chambers and through the bubble separation and froth formation chambers, as indicated by the dotted arrows at S in chamber C, and are discharged by overflow at the froth overflow level L. It will be apparent from Fig. 16 that the bubbles from the agitation chamber B1 and the first half of chamber 32 will pass upwardly into the first bubble separation and froth formation chamber C2 and will collect to form a froth that is removed at the top of the body of pulp in that chamber, and that similar conditions will prevail in the other agitation chambers and the other bubble separation and froth formation chambers, the bubbles rising from the second half of the last agitation chamber 133 passing upwardly through the last bubble separation and froth formation chamber C3 and forming a froth which is collected and removed from that chamber. As the pulp passes longitudinally ofthe tank through each bubble separation and froth formation chamber, in passing from one agitation zone to the next, additional bubbles and attached solids rise and are discharged in the froth overflowing at the top of that chamber.

The tailings, comprising the liquid and solid constituents of the pulp that are not floated and removed by overflow in the froth as described above, are discharged at the other end of the tank, principally through the upper discharge opening i i. In order to prevent undesirable accumulation of solids in the tank adjacent the tailings discharge end, a portion of the tailings may also be discharged from the last agitation chamber B3 through the lower discharge opening IS. The gate ifia shown in Fig. i may be adjusted to control the proportion of tailings discharged through the lower opening 15 and to cause the major portion of the tailings to pass upward through the central portion of the last bubble separation and froth formation chamber and over the gate Hla shown in Fig. 1, so that by adjustment of gate Ma the pulp level within the tank may be maintained at a desired level adjacent but slightly below the froth overflow level L, and a substantial distance above the bot- ,tom of the bubble separation and froth formation zones, as indicated forexample by the line L1L1 in Figs. 4 and 16.

The process may be carried out in substantially the same manner in the form of apparatus shown in Figs. 9 to 12, except for the difference in the circulatory movement of the pulp in the agitation chambers, resulting from the different location of the pulp withdrawal and pulp return openings and the diiierence in the shape of the lower portion of the tank, as brought out above in the description of that form of apparatus.

The process may also be carried out in a generally similar manner in the form of apparatus shown in Figs. 13 to 15. Fig. l? is a schematic illustration of an apparatus of this type comprising three successive agitation chambers E1, E2 and E3, and four successive bubble separation and froth formation chambers F1, F2, F3 and F4, arranged in the same manner as in Fig. 13 except that the last chambers E3 and F4111 Fig. 17 corre spond to the chambers E; and in Fig. 13. In this case also, the froth overflow troughs i8 and the baffles NH and it? are omitted in the schematic showing.

In this form of apparatus the pulp is again introduced at one end of the tank through the feed compartment Hill and opening let. The general linear movement of the pulp longitudinally of the tank, alternately through the successive agitation zones and bubble separation and froth formation zones, also follows a sinuous path extending below the upper partitions G8 and above the lower partitions ill, as indicated by the solid arrows at F", while the circulation of the pulp in each agitation chamber follows generally circular or annular paths about a vertical axis substantially centrally of that chamber, as indicated by the dotted arrows at R" in chamber E1. As before, this circulatory movement in each agitation chamber is superimposed upon the general linear now of pulp therethrough so that the actual motion of the pulp is the resultant of the two components of movement described above.

The gas bubbles with attached solid particles rise through the bubble separation and froth formation chambers to form a froth which is discharged by overflow at the froth overflow level L". This upward flow of gas bubbles and froth is indicated by the dotted arrows at S, and it will be noted that the bubbles rising from the first half of the first agitation chamber will pass upwardly into the first bubble separation and froth formation chamber F1 and will. collect to form a froth that is removed at the top of the body of pulp in that chamber. The bubbles that rise upwardly between adjacen pairs of upper partitions 98 are discharged in the form of froth at the top of the intermediate bubble separation and froth formation chambers F2 and F3, while the bubbles rising beyond the last partition 38 are similarly discharged at the top of the last chamber F4.

The tailings are discharged in the same manner as before at the other end of the tank, and the gate icfia is adjusted to maintain the desired pulp level, as indicated for. example at Lz-Jn, adjacent but slightly below the froth overflow level L".

Due to the active circulation and agitation of the pulp by pump action in each of the agitation chambers of any of the above forms of apparatus, a relatively high concentration of solids may be maintained in these chambers, which is of great advantage in promoting contact of the solid particles with added flotation agents and with gas bubbles formed by introduction of gas into the pulp. The resulting high pulp density is of added benefit in further decreasing the settling rate of individual solid particles. Furthermore,

by introducing flotation agents into the pulp in these lower zones of high solids concentration or pulp density, in which the proportion of liquid to solids is relatively low, a high concentration of such agents may be maintained in the liquid phase in these zones, thus further enhancing the effectiveness of these agents in promoting the attachment of desired solid particles to the gas bubbles.

A portion of the pulp is continually removed from each agitation chamber by the general longitudinal component of flow, and is positively directed into and caused to pass through a succeeding bubble separation and froth formation chamber. However, the transverse partitions and bafiies. as described above, prevent the circulatory agitating movement of the pulp from persisting upward into the bubble separation and froth formation chambers, so that a relatively quiescent condition is maintained therein, promoting the upward movement of gas bubbles with attached solid particles and the settling of solid particles that are not attached to gas bubbles. Due to the relatively low degree of agitation in the bubble separation and froth formation chambers, the proportion of unattached solids maintained in suspension in the liquid therein is relatively low, and the resulting low solids content or pulp density in these upper zones further promotes the settling of such solids and the formation and removal of a froth containing the gas bubbles and attached solids.

The concentration of solids in the agitation zones is dependent upon the depth of these zones of active agitation as compared with the total 9 depth of the body of pulp in the tank, which is in turn dependent upon the level of the pulp withdrawal openings in relation to the bottom of the tank and to the pulp level such as Ll-Ll or L2Lz. Thus, the concentration of pulp in the agitation zones may be controlled by placing the pulp withdrawal openings, such as 40 or I? I, M6, at the most desirable level or, in the form of apparatus shown in Figs. 9 to 12, by vertically adjusting the pulp withdrawal pipe sections 18a to adjust the level of the openings 19a. In all cases, however, these pulp withdrawal openings are within the lower agitation zones and a substantial distance below the tailings overflow lip and the pulp level.

It will be seen, therefore, that the pulp is passed through upper bubble separation and froth formation zones of relatively low pulp density, superimposed above and in unobstructed communicationwith the lower agitation zones, and that an advantageous difference in pulp density is maintained in the upper and lower zones. In conventional types of flotation apparatus the pulp density may be in the neighborhood of 20 to 30% solids, Whereas in my apparatus, if the feed pulp has a solids content within this range, pulp density of from 40 to 75% solids may be maintained in the lower agitation zones as compared with about to 15% solids in the pulp in the upper bubble separation and froth formation zones.

The consumption of air or other gas is also relatively low as compared with other forms of flotation apparatus, since only the necessary amount of gas for bubble formation and production of froth is required. The use of pump means to effect active agitation and suspension of solids in the lower agitation zones eliminates the necessity of introducing additional air or other gas for agitating or circulating the pulp.

The upward movement of gas bubbles and the formation and separation of froth by the preferred form of froth overflow means described above are illustrated schematically in Fig. 4, in which I have shown portions of two adjacent troughs I 8 defining therebetween a vertically extending passage 25, and provided at their upper ends with froth overflow lips 2'! at the froth-overflow level such as L. Due to the above described formation of the trough side walls 23, the width and cross-sectional area of the passage 25 decrease upwardly tothe restricted throat 25a and then increaseupwardly to the froth overflow level at the top of the troughs. As the gas bubbles, indicated at T1, rise through the pulp toward the restricted throat 25a they are crowded closer together and, due to their upward velocity as they rise above this restricted throat, they tend to move upward principally in the central portion of the divergent upper portion of the passage, as indicated at T2.

The level of the pulp is maintained slightly below the froth overflow level and above the restricted throat 25a, as indicated for example, at L1 -L1. The bubbles rising in the central portion of the passage between the troughs escape from the pulp and form a froth, as indicated at T3. Since the formation of this froth occurs principally in the central portion of the space between the lips 21, the froth tends to build up at this position to a height slightly above the lips 27, and then to move laterally outward toward the lips, as indicated by the arrows above the froth in Fig. 4, and to flow over the lips and into the troughs. This produces a positive discharge and overflow of the froth. Furthermore, the lateral outward movement of the froth provides a further opportunity for any unattached gangue particles that may have been entrained mechanically and carried upward by the rising bubbles to drop out of the frothand fall back into the body of pulp, as indicated by the arrows at U.

The relatively great total length of the overflow lips Zl of the transversely extending troughs I8- enables a large quantity of froth to be discharged at the top of each bubble separation and froth formation chamber, without causing the froth to pile up and flow over the lips in streams of considerable depth, as would be the case if it were attempted to discharge the same quantity of froth by overflowing only at the upper edges of the walls of the tank. This leads to the production of a froth product or concentrate of uniform high quality at a high rate, so that in an apparatus of large dimensions and high throughput capacity, the froth discharge capacity may be made commensurate with the ability of the apparatus to produce a froth by agitation and aeration of the pulp."

I claim:

1. In a froth flotation apparatus, the combination comprising: a horizontally elongated tank having pul feed means at one end, tailings discharge means at the other end, and froth dis-- charge means at the upper portion of the tank providing horizontal froth overflow lips dis tributed along the length of the tank; said tailings discharge means including means defining a tailings overflow lip near the top of the tank for maintaining therein a pulp level adjacent but slightly below the level of the froth overflow lips; a plurality of longitudinally spaced transverse lower partitions within the tank, each extending across the width of the tank and extending upward from the bottom of the tank to an intermediate level spaced a substantial distance below the level of said tailings overflow lip so as to divide the lower portion of the tank into a plurality of successive pulp agitation chambers while providing -openings above said partitions for longitudinal flow of pulp; the upper portion of the tank having walls defining a bubble separation and froth formation space therein overlying and opening directly into the upper ends of said agitation chambers for free communication therewith, and extending upwardly therefrom to said froth overflow lips; means for agitating pulp in each of said agitation chambers, comprising a pump connected to the corresponding chamber to withdraw pulp therefrom at one position and to return it thereto at a different position; the positions at which the pulp is Withdrawn from and returned to each agitation chamber by the corresponding pump, and the direction in which the return connection of each pump opens into the corresponding agitation chamber, being such as tomaintain active circulation of pulp in the respective agitation chambers, in paths that extend into the upper portions of said chambers but below said bubble separation and froth formation space; and means for introducing gas under pressure into the pulp at a plurality of distributed positions within the upper portion of each agitation chamber, within said paths of pulp circulation and below said bubble separation and froth formation space.

2. In a flotation apparatus, the combination as set forth in claim 1, and also comprising a plurality of complementary transverse upper partitions disposed within the tank intermediate the lower partitions, each extending across the width of the tank and extending from the level of said froth overflow lips downward to below the level of the upper ends of the lower partitions so as to divide the upper portion of the tank into a plurality of bubble separation and froth formation chambers while providing openings below the upper partitions for longitudinal flow of pulp; said means for introducing gas into the pulp being located at positions adjacent the level of the lower ends of said transverse upper partitions.

3. In a froth flotation apparatus, the combination comprising: a horizontally elongated tank having pulp feed means at one end, tailings discharge means at the other end, and froth discharge means at the upper portion of the tank providing horizontal froth overflow lips distributed along the length of the tank; said taillngs discharge means including means defining a tailings overflow lip near the top of the tank for maintaining therein a pulp level adjacent but slightly'below the level of the froth overflow lips;

a plurality of longitudinally spaced transverse lower partitions within the tank, each extending across the width of the tank and extending upward from the bottom of the tank to an intermediate level spaced a substantial distance below the level of said tailings overflow lip so as to divide the lower portion of the tank into a plurality of successive pulp agitation chambers while providing openings above said partitions for longitudinal fiow of pulp; the upper portion of the tank having walls defining a bubble separation and froth formation space therein overlying and opening directly into the upper ends of said agitation chambers for free communication therewith, and extending upwardly therefrom to said froth overflow lips; means for agitating pulp in each of said agitation chambers, comprising a pump connected to the corresponding chamber to withdraw pulp therefrom at one position and to return it thereto at a different position; and means for introducing gas under pressure into the pulp at a plurality of distributed positions within the upper portion of each agitation chamber and below said bubble separation and froth formation space; the inlet of each pump being connected to the corresponding agitation chamber at a pulp withdrawal opening located a substantial distance above the bottom thereof and a substantial distance below the level of the tailings overflow lip, and the outlet of each pump being connected to the corresponding agitation chamber at a pulp return opening adjacent the bottom thereof.

4. In a flotation apparatus, the combination as set forth in claim 3, in which the connection from the outlet of. each pump to the pulp return openil'lg opens into the agitation chamber in a direction substantially parallel to the bottom wall of the tank.

5. In a flotation apparatus, the combination as set forth in claim 3, in which the connection from the outlet of each pump to the pulp return opening opens into the agitation chamber in a direction transverse to the length of the tank so as to cause circulation of pulp within the agitation chamber about an axis extending longitudinally of the tank.

6. In a flotation apparatus, the combination as set forth in claim 3, in which said gas introducing means communicates with the interior of the tank at positions adjacent the level of the pulp withdrawal opening of each agitation chamber.

7. In a froth flotation apparatus, the combination comprising: a horizontally elongated tank having pulp feed means at one end, failings discharge means at the other end, and froth discharge means at the upper portion of the tank providing horizontal froth overflow lips distributed along the length of the tank; said tailings discharge means including means defining a tailings overflow lip near the top of the tank for maintaining therein a pulp level adjacent but slightly below the level of the froth overflow lips; a plurality of longitudinally spaced transverse lower partitions within the tank, each extending across the width of the tank and extending upward from the bottom of the tank to an intermediate level spaced a substantial distance below the level of said tailings overflow lip so as to divide the lower portion of the tank into a plurality of successive pulp agitation chambers while providing openings above said partitions for longitudinal flow of pulp; the upper portion of the tank having walls defining a bubble separation and froth formation space therein overlying and opening directly into the upper ends of said agitation chambers for free communication therewith, and extending upwardly therefrom to said froth overflow lips; means for agitating pulp in each of said agitation chambers, comprising a pump connected to the corresponding chamber to withdraw pulp therefrom at one position and to return it thereto at a different position; and means for introducing gas under pressure into the pulp at a plurality of distributed positions within the upper portion of each agitation chamber and below said bubble separation and froth formation space; said froth discharge means comprising a plurality of troughs extending inward from each side of the tank and transverse to the length of the tank; each trough extending downward within the bubble separation and froth 

